1. Field of the Invention
The present invention relates to an electrophotographic photoconductor which comprises an organic compound with high charge transportability as a charge-transporting material, and to an image-forming apparatus comprising the same.
2. Description of the Related Art
Recently, organic photoconductive materials have been widely researched and developed. They have been used for electrophotographic photoconductors (also referred to simply as “photoconductors”), and are now beginning to be applied to electrostatic recording devices, sensor materials, organic electroluminescent (EL) devices, etc.
Organic photoconductors, in which organic photoconductive materials are used, are utilized not only in the field of copiers, but also in the fields of printing plates, slide film and microfilm, for which conventional photographic technology has been employed. Further, they are applied to high-speed printers with the use of a laser, a light emitting diode (LED), a cathode ray tube (CRT) or the like as a light source.
Thus, there is a high level and wide range of demands on organic photoconductive materials and organic photoconductors.
Conventionally, as an electrophotographic photoconductor, an inorganic photoconductor has been widely used, which contains, as its active material, an inorganic photoconductive material such as selenium, zinc oxide or cadmium.
Although having basic properties for an electrophotographic photoconductor to some degree, an inorganic photoconductor has problems of difficulties in forming its photosensitive layer, poor plasticity of the layer, high production costs, and the like. In addition, generally inorganic photoconductive materials are so toxic that they are limited in terms of production and handling.
In contrast, organic photoconductors have various advantages of the photosensitive layer being easy to form, flexible, lightweight, and highly transparent. They also can be readily designed to have good sensitivity to a wide range of wavelengths by appropriate sensitization. Accordingly, in recent years, electrophotographic photoconductors have been developed mainly as organic photoconductors.
Although early organic photoconductors had some drawbacks in sensitivity and durability, these drawbacks have been significantly overcome by function-separated photoconductors in which the charge generation and transport functions are respectively served by different materials.
Such function-separated photoconductors have advantages to be relatively easily produced to have any desired properties since a charge-generating material responsible for the charge generation function and a charge-transporting material responsible for the charge transport function each can be selected from a wide range of materials.
As a charge-generating material used in a function-separated photoconductor, many materials have been examined, such as phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes and pyrylium salt dyes, and various materials have been proposed that exhibit high light resistance and high charge-generating ability.
As a charge transporting material, a variety of compounds are known, such as pyrazoline compounds (see, for example, Japanese Patent Publication No. Sho 61 (1986)-189547-A), hydrazone compounds (see, for example, Japanese Patent Publication No. 2000-143654-A), triphenylamine compounds (see, for example, Japanese Patent Publication No. Sho 58 (1983)-32372-B), stilbene compounds (see, for example, Japanese Patent Publication Nos. Sho 58 (1983)-198043-A and Hei 2 (1990)-190862-A) and enamine compounds (see, for example, Japanese Patent Publication No. Hei 2 (1990)-51162-A).
Charge-transporting materials are required: (1) to be stable to light and heat; (2) to be stable to ozone, nitrogen oxides (NOx), nitric acid and the like generated by a corona for charging a photoconductor surface; (3) to have high charge-transportability; (4) to be highly compatible with an organic solvent and a binder resin; and (5) to be easy to produce, and inexpensive. Conventional charge-transporting materials as described above meet some, but not all, of the requirements at a high level.
Of the above requirements, (3) “to have high charge-transportability” is the most important. This is because the charge-transportability of a charge transporting material used for a photoconductor needs to be sufficiently high to ensure a high photoresponsiveness of the photoconductor's surface layer formed by dispersing the material with a binder resin.
When a photoconductor is used in a copier, a laser beam printer or the like, the photoconductor's surface layer is scraped off by one or more contact members such as a cleaning blade and a charging roller. In order to increase the durability of copiers, laser beam printers or the like, therefore, it is necessary that the surface layer is hard to be scraped off by such a contact member, i.e., it has high printing durability.
If the content of the binder resin in the charge transport layer as a surface layer is increased so as to make the layer more durable, the photoresponsiveness of the layer decreases. This is because conventional charge-transporting materials have low charge-transportability and therefore the charge-transportability of the charge-transport layer further decreases when the charge-transporting material content decreases as a result of the increase of the binder resin content.
If the photoresponsiveness of a photoconductor is poor, its residual surface potential increases and therefore the photoconductor is repeatedly used without sufficiently attenuating the surface potential. In the photoconductor, the surface charge in the area exposed to light is not sufficiently erased, resulting in a rapid deterioration in image quality. This is a reason why charge transporting materials for photoconductors are required to have high charge-transportability sufficient to ensure high photoresponsiveness.
Since the time from the exposure to light to the image development in a high-speed process is short, photoconductors used in such a process are required to have high photoresponsiveness. This is another reason why charge transporting materials are required to have high charge-transportability, which contributes to high light responsiveness, as explained above.
In addition, since it is now demanded that electrophotographic apparatus such as digital copiers and printers be downsized and operable at a higher speed, photoconductors are required to be high sensitive enough to be operable in such a high-speed. Furthermore, photoconductors are also required to be highly reliable so as not to decrease in sensitivity under low temperature environments and to show a little variation in their properties under various environments. These are other reasons why charge-transporting materials are required to have high charge-transportability.
Molecular designing technique has been used for developing such charge-transporting materials that meet the above requirements. As a result, compounds having hydrazone and styryl structures which form largely extended conjugated systems in the basic structures, and bis-enamine compounds have been proposed as superior charge-transporting materials (see, for example, Japanese Patent Publication Nos. Hei 5 (1993)-66587-A, Hei 6 (1994)-348045-A, 2000-235272-A). However, these compounds decrease in sensitivity under low temperature environments. Thus, there is a demand to develop such compounds that show sufficient charge-transportability under low temperature environments.